Process for producing a rotationally symmetric hollow part and hollow part produced thereby

ABSTRACT

A process for producing a rotationally symmetric hollow part, especially a shaft, including: providing a bar-shaped solid material involving the steps of heating the solid material substantially to forging temperature; cross wedge rolling of the solid material so as to cause weakening in the core zone of the solid material; and inserting at least one rotating mandrel, driven at a predetermined velocity, substantially along the central axis of the cross-wedge-rolled solid material, thus creating a through-hole and to a rotationally symmetric hollow part, especially a shaft.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for producing a rotationally symmetric hollow part, especially a shaft, as well as to a cross-wedge-rolled rotationally symmetric hollow part.

2. Description of Related Art

It has become increasingly common to produce rotationally symmetric hollow parts that may also serve as a preform for further forming, e.g., stepped shafts, and especially gear shafts, by means of cross wedge rolling. This is performed on flat or round jaw machines. As a result of the rolling process, the outer zones of the cross-wedge-rolled shafts are hardened. The use of solid material results in high weight, which is especially undesirable when such shafts are used in automotive industry. This is why gear shafts are produced or welded with high costs from hollow shafts by using rotary swaging machines, or the shafts are mechanically wrought (gun drilled).

From German Patent Application DE 10 2006 031 564 A1 and corresponding U.S. Patent Application Publication 2009/0312110 A1, the following steps are taken for the production of a hollow rotationally symmetric part:

-   -   providing bar-shaped solid material;     -   heating the solid material substantially to forging temperature;     -   cross wedge rolling of the solid material, thus causing         weakening in the core zone of the solid material; and     -   inserting at least one mandrel substantially along the central         axis of the cross-wedge-rolled solid material, thus creating a         through-hole.         However, the resulting hollow parts, especially shafts, still         have the drawback that the inner bore has rungs or helicoidal         threads, which are created when using pressure mandrels.         Finishing work on the inner contour of the hollow shaft is         costly and also leads to weakening of the fiber orientation         within the shaft. The fact that the shaft is weakened also means         that more material is necessary to achieve the same strength,         i.e., the weight of the shaft is increased. This is undesirable         in view of lightweight construction requirements. Besides, a         non-round inner contour leads to considerable problems with         centering the shaft, requires extensive refinishing work and         inhibits unproblematic, smooth running.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to create a process for producing a rotationally symmetric hollow part, so that hollow parts with a smoother inner contour can be produced.

According to the invention, this object is achieved by using rotating mandrels, which can be driven in an appropriate manner to allow controlling of a relative motion between the mandrel and the solid material to be indented so that the inner contour is substantially smooth and the thickness of the walls is substantially constant in every cross section. Thus, it becomes possible to produce a smoother-running hollow part which requires only minor or no balancing work.

Further, the invention relates to a cross-wedge-rolled rotationally symmetric hollow part, especially a shaft, which can be a gear shaft, cam shaft, drive shaft, output shaft, starter clutch shaft, hollow shaft or a preform for other forming parts, which is produced according to the process.

Thus, by inserting the driven rotating mandrels into the inner zone which is weakened during the rolling process as the crystal lattice of the forgeable material is weakened by the rolling movement, a smoothed-out through-hole can be achieved, so that a rotationally symmetric hollow part, such as a hollow shaft, can be easily produced with high precision. This weakening of the bar core during cross wedge rolling is also known as the Mannesmann effect. Due to the high external pressure on the bar during cross wedge rolling, the outer layer of the bar-shaped material is hardened, so that widening the walls becomes easier. By inserting the at least one driven rotating mandrel, a high workpiece precision can be attained as the material remains formed by the exterior forming tools, while the hardening resulting form rolling leads to a shaft with corresponding load capacity.

The at least one mandrel can have any shape, such as a tooth shape, a hexagon, a spin profile, etc. It is also advantageous to have a rounded mandrel front part, so that a smoother inner contour of the hollow part can be obtained. A rotational drive for the mandrel, which is preferably controlled, is well known to a person skilled in the art.

Thus, the process can easily be performed for high piece numbers, where, thanks to the deployed forming technique, a shape is attained that practically corresponds to the final shape, so that there is substantially no need for further finishing of the resulting workpiece, as it requires only minor or no balancing work.

Substantial savings can be realized not only for the workpiece itself, i.e., by saving material, but also by reducing production costs through the simplification of the complex balancing process, which so far has made costly finishing steps necessary.

The hollow shaft has a reduced weight compared to conventional shafts made of solid material, while at the same time maintaining their strength. By inserting a driven and controlled rotating mandrel, the material in the core is displaced outward in a controlled manner. With the material thus pressed against exterior forming tools, a high workpiece precision can be attained.

It can be advantageous to insert two controlled driven rotating mandrels along the ends of the bar-shaped solid material. In this way, the path of the mandrel is shortened and a higher cycle time can be attained. Here, the mandrels are only inserted to the point where they are almost touching each other. In the course of the process, one mandrel is pulled back while the second mandrel is inserted further along an overlap zone.

The mandrels can be advantageously inserted so that they are rotated simultaneously by means of a rotational drive. However, it is also possible to insert the mandrels so that they rotate asynchronously.

A typical shaft according to the invention which is applied as a main gear shaft or layshaft has a diameter of approximately 30 to 200 mm, preferably of 60-150 mm. Of course, higher or lower diameters can also be realized. The shaft advantageously consists of a ductile wrought alloy, such as a 16MnCrS4, 20MnCr5, 20MoCrS4 steel, an aluminum or magnesium alloy, precipitation-hardened steels or any conventional steel known to a person skilled in the art.

In the following, the invention is described in more detail by using an exemplary embodiment of a hollow shaft, to which, however, it is by no means limited, as well as the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a bar-shaped solid starting blank material;

FIG. 2 is a cross-sectional view of the solid material during cross wedge rolling;

FIG. 3 is a cross-sectional view of a shaft during cross wedge rolling just prior to insertion of rotating mandrels;

FIG. 4 rotating mandrels of the shaft with two blind bores while the rotating mandrels are inserted;

FIG. 5 a cross section of a shaft with a through-hole;

FIG. 6 is a schematic view of a cross wedge rolling machine; using the example of a flat jaw machine with material feed which provides positional strength during flaring;

FIG. 7 is a schematic view of the front of the cross wedge rolling machine of FIG. 6; and

FIG. 8 is a schematic representation of a cross wedge rolling step, in which the rotating mandrels are inserted into the solid material.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a bar 1 made of solid material that is heated up to forging temperature. FIG. 2 schematically shows how it has been formed into a cross-wedge-rolled shaft with different diameters. During rolling, the bar 1 is moved with strong force between tools 12, 14, so that the material in an outer zone 4 hardens while the core 3 becomes brittle and cracks. The tool 12, 14 forms the exterior of the shaft 2, which is already close to its final shape.

FIG. 3 shows two movable mandrels 5, 6, which are driven to rotate, being centrally inserted into the shaft 2 from opposite ends of the shaft 2 in an axial direction along the core 3, which has been weakened through the Mannesmann effect. The mandrels 5, 6 are pushed forward to the point where they are just short of touching each other. In this way, the material of the shaft is pressed more strongly outward against the tools 12, 14, which results in a precise outer contour.

FIG. 4 shows a cross section of the transversely-rolled shaft 2 in the first form. On both front sides, a blind hole 8, 8 has been created by the mandrels.

FIG. 5 shows a cross section of a transverse-rolled shaft 2, which has a through-hole created by inserting the mandrels 5, 6 in an overlapped manner. To create this through-hole, one of the mandrels 5, 6 is pulled back from an overlap zone of the mandrel paths, while the corresponding other mandrel is rotated and inserted across the overlap zone, so that a smooth through-hole 9 is created. For the through-hole to be smooth, in a further step, the mandrel 5 which creates the through-hole can be pulled back again and the withdrawn mandrel then driven across the overlap zone.

Thereby, a cross-wedge-rolled hollow shaft is created, for which also bigger diameters are conceivable depending on the size of the machine. Typical dimensions of the shaft are diameters from 30 to 200 mm, and preferably 60-150 mm. Appropriate materials are ductile materials, such as malleable wrought alloys. However, the alloys are by no means restricted to iron alloys—appropriate non-iron alloys or alloys having lower iron content, such as ductile aluminum, titanium or magnesium alloys, may also be used.

For a better understanding of the process, FIG. 6 shows a schematic representation of a cross wedge rolling machine 10. A bar 1 is supported at opposite sides by material supports 16, 18 in a cage-like arrangement together with two exterior tools 12, 14 which are arranged opposite each other. The outer tools 12, 14, are arranged perpendicular to the material supports 16, 18. A tool 12 with the tool tray 13 is arranged in a substantially static manner, while the second tool 14 with the tool tray 15 and the material supports 16, 18 moves up and down or back and forth in two linear directions with the rolling bar material. The workpiece is acted upon from both sides by the tools 12, 14 with high force, so that a cross wedge rolled shaft 2 is created from the bar part 1.

Through the reciprocating movement of the tool 14, the outer zone 4 of the shaft is hardened, while the negative relief of the tool 12, 14 is transferred as a positive form to the shaft 2 and the shaft core is weakened.

FIG. 7 shows a schematic side view of this cross wedge rolling machine 10, with a wedge-shaped tool 12 exerting force on the shaft 2, and the shaft 2 being formed by material support 16 and tool 14.

FIG. 8 shows a schematic representation of a cross wedge rolling step, in which the mandrels are inserted rotationally into the solid material and support the inner contour, while the outer contour of the tube is formed into a flange 20. The rotating mandrels, which in this example have a rounded tip, support the material as it flows into the flange 20, with the arrows representing the force of the forming tool acting on the material to be formed. It can be seen that the mandrels support the material during the local pressure exertion through the forming tool in order to minimize the undesirable material flow into the bore. The excess material 22 produced as a result of the forming process is smoothed out during the further procedure by pulling out of the mandrels, so that a smooth inner bore is created.

While the invention has been described by way of preferred embodiments, various alternative designs and embodiments for practicing the invention, as defined in the accompanying claims, will be evident to persons skilled in the art. 

What is claimed is:
 1. Process for producing a rotationally symmetric hollow part, comprising the steps of: providing a bar-shaped solid material; heating the solid material substantially to forging temperature; cross wedge rolling of the solid material in a manner causing weakening in a core zone of the solid material; and inserting at least one rotating mandrel driven at a predetermined velocity substantially along a central axis of the cross-wedge-rolled solid material in a manner creating a through-hole.
 2. Process according to claim 1, wherein said at least one rotating mandrel comprises two rotating mandrels, each of which inserted into a respective one of opposite ends of the bar-shaped solid material.
 3. Process according to claim 2, wherein the mandrels are inserted simultaneously.
 4. Process according to claim 2, wherein the mandrels are inserted asynchronously.
 5. Cross-wedge-rolled rotationally symmetric hollow part in the form of one of a gear shaft, cam shaft, drive shaft, output shaft, starter clutch shaft, hollow shaft and a preform for other forming parts produced by the process of claim
 1. 6. Cross-wedge-rolled rotationally symmetric hollow part according to claim 5, wherein the part has a diameter of approximately 30 to 200 mm.
 7. Cross-wedge-rolled rotationally symmetric hollow part according to claim 5, wherein the part has a diameter of approximately 60-150 mm.
 8. Cross-wedge-rolled rotationally symmetric hollow part, to claim 5, wherein the part is made of a ductile wrought alloy.
 9. Cross-wedge-rolled rotationally symmetric hollow part, according to claim 5, wherein the part is made of a material from the group comprising 16MnCrS4, 20MnCr5, and 20MoCrS4 steel.
 10. Cross-wedge-rolled rotationally symmetric hollow part, according to claim 5, wherein the part is made of a material from the group comprising a non-iron alloy, aluminum alloy, titanium alloy and magnesium alloy. 